Alga are widely distributed in aquatic ecosystems and play an important role in arsenic (As) bioavailability and biogeochemical cycling. As one of important freshwater algae, Microcystis aeruginosa have been identified for their high heavy metal and As accumulation rates and their possible usage for water treatment. Ongoing research shows the potential of this organism for removing As as well as important process parameters related of the organism for accumulation and detoxification by means of transformation to less toxic inorganic or methylated As species. Additionally, when using coagulation technology to remove As-loaded algae from the water phase, there parameters should be optimized including pH, coagulant types, arsenic levels. Meanwhile, additional budget should be allocated to the management of the toxic sludge produced since precipitated M. aeruginosa may to a certain extent release As again into the water phase (in situ or ex situ disposal). Thus, environmental safety of the coagulation technology itself should be assessed prior to its use, which has not yet been done so far. In particular, changes in As content and As speciation in As-loaded M. aeruginosa were not addressed clearly after in-situ or ex-situ disposal in water phases after algal coagulation and precipitation. We thus investigated changes of As concentrations and its species in the in-situ treated algae water by optimal coagulation and As release kinetics from its subsequent living and dead precipitated algae after ex-situ disposal. Results showed that the optimal coagulation treatment for removing algae was 25 mg/L PACl, 1.0 µM arsenate (As(V)), and pH 6.0, whereby we attained greater than 99.5% algal removal efficiency. Moreover, in the in-situ treated water, total dissolved As slowly decreased as standing for 6 days, but arsenite (As(III)) largely increase after 3 days standing. Additionally, potential risks arised from As release in short-term duration (24 h) from both living and dead precipitated algae at 1.0 μM As(V) pre-exposure due to their higher release rate constants. Similarly, As release in long-term (6 d) duration from dead alga at 10.0 µM As(V) pre-exposure also resulted in potential risks. Moreover, As release from precipitated M. aeruginosa depends not only on As content in the algae but also on the status – living or dead – of the algae while being disposed ex-situ. This study offers insights into the appropriate disposal of precipitated algae with As-contamination when either bloom algae exposed As or being used for bioremediation of As contaminated water. This study was jointly supported by Major Science and Technology Program for Water Pollution Control and treatment (2017ZX07101001-02), the National Nature Science Foundation of China (project nos. 41401552, 41271484, and 41773100), the Nature Science Foundation of Fujian Province (2016J01691 and 2017Y0081) and Erasmus Mundus International Master program IMETE.